For instance: after downloading "Sylvie and Bruno" (some 18MB) I did a "tweak epub" and batch conversion through IrfanView, keeping names but *to gray-scale, *largest dimension, 500 px. The result, to my taste, is just beautiful

Ah, but the pictures are already grayscale, it's just that the grays appear almost only at the edges of the black lines, since the original illustrations are only black ink on white paper.

You are welcome to resize the images if it works better for you. I preferred to include them at a larger size/resolution, hoping that some day they will be used with larger screens or devices with better resolution, and to allow zooming the images where possible.

Note that, when you resize all images to a fixed maximum size, you are losing the relative proportion between images. It might be better to resize to a given percentage of the original size.